Most GST notices are not about fraud. They come from small, repeatable filing errors that the portal now catches on its own. GSTR-1, GSTR-2B, GSTR-3B, e-invoices and e-way bills are matched by machine every month, and gaps go straight out as DRC-01B or DRC-01C intimations, ASMT-10 scrutiny notices, or a blocked e-way bill. Each one costs interest, a late fee, or an afternoon spent replying.

These are the fifteen mistakes we see most often in 2026, grouped by where they happen, with the fix for each. Every figure here can be checked with a free tool, linked as we go.

Outward supplies and GSTR-1

1. Paying less tax in GSTR-3B than GSTR-1 shows

GSTR-1 says what you sold and GSTR-3B says what you paid tax on. When the GSTR-3B liability is lower, Rule 88C sends a DRC-01B intimation. You must pay the gap or explain it before your next GSTR-1 can be filed. Since the July 2025 tax period, the liability auto-filled into GSTR-3B from GSTR-1/1A can no longer be edited downwards in GSTR-3B. Fix GSTR-1 through GSTR-1A before you file 3B, not after.

Fix: reconcile the two every month, not every March. Our GSTR-9 summary tool puts all twelve months side by side and works out the DRC-03 amount and interest for any month that was short-paid.

2. Reporting a B2B sale as B2C

If you leave out the buyer's GSTIN, the invoice goes into the B2C tables and never reaches your customer's GSTR-2B. They lose the credit and you lose the customer's trust. Check the GSTIN when you raise the invoice. Amend wrong entries in the next GSTR-1, and do it before 30 November after the year ends.

3. Wrong HSN codes or too few digits

Table 12 of GSTR-1 needs 4-digit HSN codes up to ₹5 crore turnover and 6-digit codes above that, reported separately for B2B and B2C. A wrong code usually means a wrong rate, and that is especially likely now that many goods moved slabs after GST 2.0. Look the code up rather than copying last year's. The HSN code finder shows the current rate.

4. E-invoices and e-way bills that don't match GSTR-1

E-invoice data from the IRP fills GSTR-1 automatically. If you then add the same invoice by hand, or cancel it on the IRP but not in your books, the totals drift apart. Treat the IRP as the source of truth, and reconcile e-way bill values against GSTR-1 for every month with heavy movement.

Input tax credit

5. Claiming credit that isn't in GSTR-2B

Since January 2022 you can claim only what appears in GSTR-2B. The old provisional margin is gone. A claim above GSTR-2B triggers a DRC-01C under Rule 88D. If the excess was used to pay tax, it also carries 18% interest under Section 50(3). See exactly where you stand month by month on the ITC reconciliation dashboard.

6. Ignoring the Invoice Management System

Invoices you neither accept nor reject in IMS are treated as accepted when GSTR-2B is generated. If an invoice is wrong (a duplicate, a wrong amount, a purchase that never happened), reject it before 2B is generated. Otherwise the credit flows in and you will have to reverse it later. Our IMS guide walks through the month-end routine.

7. Forgetting the reversals

• Rule 37: credit on any invoice not paid to the supplier within 180 days must be reversed, and can be reclaimed once you pay.
• Rule 37A: credit must be reversed if the supplier has not filed GSTR-3B for that period by 30 September of the following year.
• Rules 42 and 43: common credit must be split when you also make exempt supplies.
• Section 17(5): blocked credits such as most motor vehicles, food and beverages, and personal-use items must never be claimed.

8. Missing the Section 16(4) deadline

Credit for FY 2025-26 can be claimed only until 30 November 2026, or the date you file the annual return if that comes first. After that, any invoice your supplier uploaded late, or that you simply forgot, is lost for good. Look for unclaimed GSTR-2B credit in October, not December.

9. Mishandling reverse charge

Goods transport, legal services, renting by an unregistered person to a registered business and the other notified supplies put the tax on you. Two errors are common: not paying the RCM liability at all, and claiming the matching ITC before paying it in cash. The order matters. You pay RCM through the cash ledger, then claim the credit. The RCM calculator keeps both sides straight.

Payments, deadlines and late fees

10. Paying under the wrong tax head

Charging CGST + SGST on an interstate sale (or IGST on an intrastate one) is not fixed by moving money between heads. You pay the correct tax and claim a refund of the wrong one under Section 77. Decide the place of supply before you invoice. The place of supply tool settles the tricky cases.

11. Assuming late fee is capped at ₹10,000

The real GSTR-3B and GSTR-1 caps depend on last year's turnover: ₹2,000 up to ₹1.5 crore, ₹5,000 up to ₹5 crore and ₹10,000 above. Nil returns pay ₹20 a day, capped at ₹500. GSTR-9 has its own slabs. Late fee and interest must be paid in cash, never from ITC. Work out both in one step with the late fee and interest calculator.

12. Skipping nil returns

No sales this month still means a return. A missed nil GSTR-3B costs a late fee. Two missed periods also block your e-way bills under Rule 138E, and six months of non-filing can lead to cancellation. A nil return takes two minutes by SMS. See how to file a nil return.

13. Leaving old returns for "later"

From July 2025 the portal will not accept GSTR-1, GSTR-3B, GSTR-4 or GSTR-9 returns more than three years after their due date. A business that stops filing and plans to catch up eventually may find the window closed for good. Old gaps then become a matter for assessment rather than something you can fix yourself.

Annual return and scheme choice

14. Treating GSTR-9 as a copy-paste of the monthly returns

GSTR-9 reports outward supplies on the GSTR-1 basis (Table 4) and tax paid on the GSTR-3B basis (Table 9), and it compares claimed ITC with GSTR-2B (Table 8). Officers read those differences first. Any short payment you find while preparing it goes through DRC-03 with interest. It cannot be paid inside GSTR-9. Use the GSTR-9 checklist before the 31 December due date.

15. Getting the composition scheme base wrong

Composition dealers make their own version of these mistakes. Manufacturers pay 1% on total turnover, exempt supplies included, while traders pay 1% only on taxable supplies. Reverse-charge purchases are still taxed at the full rate. And crossing ₹1.5 crore mid-year (₹50 lakh for service providers) takes you out of the scheme on that day, not at year end. The composition tax calculator handles all three.

A monthly routine that prevents most of these

1. Before the 11th: reconcile sales books with e-invoices and file GSTR-1. Use GSTR-1A for anything missed.
2. On the 14th: act on every invoice in IMS (accept, reject or keep pending), then check GSTR-2B against your purchase register.
3. Before the 20th: file GSTR-3B with liability matching GSTR-1, ITC no higher than GSTR-2B, and RCM paid in cash.
4. Each quarter: review Rule 37 and Rule 37A reversals and chase suppliers who have not filed.
5. Each October: sweep for unclaimed credit and pending credit notes before 30 November.

Put the dates in your calendar from the GST due dates page.

Frequently Asked Questions

Related free tools

• Late Fee & Interest Calculator: what a late return really costs
• ITC Reconciliation Dashboard: books vs GSTR-2B vs GSTR-3B
• GSTR-9 Summary Generator: annual GSTR-1 vs 3B check
• GSTR-2B Reconciliation: invoice-level matching
• Composition Tax Calculator: CMP-08 by quarter